Enhanced magnetization drift velocity and current polarization in (CoFe)$_{1-x}$Ge$_x$ alloys

We present measurements of current spin polarization and magnetization drift velocity in (CoFe)$_{1-x}$Ge$_x$ alloys $(x \leq 0 \leq 0.3)$, using a spin wave Doppler technique where spin wave transmission is measured between fixed-wavevector antennas coupled to current-carrying wires [1,2]. In a current density $J$, the transmission resonance frequency is shifted by $\Delta f = kv/2\pi$, where $v=Jg\mu_{\rm B}P/(2eM_{\rm s})$ is a magnetization drift velocity. Measurement of $\Delta f$ allows calculation of $v$ and current spin polarization $P$. With increasing Ge concentration, $v$ increases dramatically from (3.1 $\pm$ 0.2) m/s for CoFe to (8.2 $\pm$ 0.6) m/s for (CoFe)$_{0.7}$Ge$_{0.3}$ ($J= 10^{11}$ A/m$^2$). We attribute this increase in drift velocity primarily to decreased magnetization. The current polarization increases from 0.84 $\pm$ 0.04 for CoFe and reaches a maximum of 0.95 $\pm$ 0.05 at approximately 25\% Ge.\\[4pt] [1] V. Vlaminck and M. Bailleul, Science, 322, 410 (2008)\\[0pt] [2] M. Zhu, C. L. Dennis and R. D. McMichael, Phys. Rev. B. 81, 140407R (2010).